Understanding the Effect of Precipitation Process Variables on Hardwood Lignin Characteristics and Recovery from Black Liquor

Abstract

Lignin has an enormous valorization potential as a sustainable raw material. The most common source of industrial lignin is black liquor, a byproduct from the kraft process. However, the variability in color and chemical characteristics of precipitated lignin is a constituent, hindering its use as a raw material. The structural complexity and heterogeneity of the lignin molecule make it challenging to determine the underlying mechanisms behind the chemical and physical changes that occur during precipitation. This work used a statistical design of experimental approach in Minitab to study how fractionation process variables influence the lignin mass yield and characteristics. The specific phenomena governing hardwood lignin solubility and precipitation in black liquor were investigated, and the relationships between chemical and physical properties and lignin color were analyzed. We found a positive correlation between temperature, mass yield, molecular weight, and dark coloration. For pH < 4, condensation of molecular fragments and sorption onto larger chains seem to drive the precipitation process.

Oxidation of Various Kraft Lignins with a Bacterial Laccase Enzyme

“…The samples precipitated at higher pH are of high ash content (HA), while those precipitated at lower pH are of medium (MA) to low (LA) ash content. This is due to the degree of protonation, which is lower at higher pH values leading to a fully protonated KL at pH 3 [ 19 , 48 , 49 ].…”

“…It is known that the grade of purity and the size of the KL molecules are influenced by the precipitation pH used [ 7 , 19 , 50 , 51 ]. During kraft pulping the lignin molecules are degraded into smaller oligomers of low molecular weights between 2 to 200 kDa [ 18 , 52 ].…”

“…Under these harsh conditions, the actually recalcitrant lignin is dissolved, indicated by a color change of the liquor from white to black [ 10 ]. The pH of this black liquor typically ranges from 13 to 14, where the functional groups of lignin, such as methoxy, hydroxyl, carbonyl and carboxyl groups, are present in their deprotonated and thus dissolved form, stabilized through electrostatic repulsion [ 19 ]. However, the black liquor not only contains lignin but also some impurities like residual cellulose and hemicellulose, inorganics and other compounds, simply referred to as ash hereinafter.…”

Innovative Sustainable Wood Preservatives From Pulp and Paper Industry Byproduct

“…The kraft lignin utilized in this study was precipitated from a mixed-hardwood black liquor obtained from Verso (Quinnesec, MI) through the LignoBoost® process to a final pH of 4.2 at 70 ºC. The lignin was vacuum freeze-dried (FreeZone 4.5L, 77500/77510, Labconco) for 18 hours at a temperature of -40 °C [218]. The characterization methodology can be found in the supplemental document.…”

“…Materials. Kraft lignin (KL) was recovered from hardwood black liquor according to our previous work [218]. The black liquor was precipitated to a final pH of 4.2 at 70 °C.…”